A variable frequency controller has circuit characteristics of a three-phase-six-arm circuit. If a power switch of the three-phase-six-arm circuit fails, an arm may be open or short so that the three-phase-six-arm circuit may not operate normally. In a failure state of a power module, a large current caused by a short circuit of an up arm or a down arm may be generated. The large current damages the power module and causes the variable frequency controller output abnormally. For example, if the up arm is short and the down arm still works normally, an over-current will be generated.
When the power module is damaged, the output power will be abnormal. If the diagnosis time is too long, the power module will be lead to a second damage, which causes the overall system being at risk. If the time of the failure state is reduced, a possibility of the second damage may be reduced.